Artificial disc replacements with natural kinematics

ABSTRACT

This invention improves upon prior aft total disc replacements (TDRs) by more closely replicating the kinematics of a natural disc. The preferred embodiments feature two or more fixed centers of rotation (CORs) and an optional variable COR (VCOR) as the artificial disk replacement (ADR) translates from a fixed posterior COR that lies posterior to the COR of the TDR to facilitate normal disc motion. The use of two or more CORs allows more flexion and more extension than permitted by the facet joints and the artificial facet (AF). AF joint-like components may also be incorporated into the design to restrict excessive translation, rotation, and/or lateral bending.

CROSS REFERENCE TO RELATED APPLICATIONS

This present application claims the benefit of U.S. Provisional PatentApplication 60/374,747, filed on Apr. 23, 2002, U.S. Provisional PatentApplication 60/445,958 filed on Feb. 7, 2003, U.S. Provisional PatentApplication 60/447,642 filed on Feb. 24, 2003. This application is acontinuation of Ser. No. 10/420,423, filed Apr. 22, 2003, now U.S. Pat.No. 6,706,068, granted Mar. 16, 2004. The disclosures of saidapplications are incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates generally to artificial disc replacements (ADRs)and, more particularly, to ADRs facilitating more natural kinematics.

BACKGROUND OF THE INVENTION

Many spinal conditions, including degenerative disc disease, can betreated by spinal fusion or through artificial disc replacement (ADR).ADR has several advantages over spinal fusion. The most importantadvantage of ADR is the preservation of spinal motion. Spinal fusioneliminates motion across the fused segments of the spine. Consequently,the discs adjacent to the fused level are subjected to increased stress.The increased stress increases the changes of future surgery to treatthe degeneration of the discs adjacent to the fusion. However, motionthrough an ADR also allows motion through the facet joints. Motionacross arthritic facet joints could lead to pain following ADR. Somesurgeons believe patients with degenerative disease and arthritis of thefacet joints are not candidates for ADR.

Current ADR designs do not attempt to limit the pressure across thefacet joints or facet joint motion. Indeed, prior art ADRs generally donot restrict motion. For example, some ADR designs place bags ofhydrogel into the disc space which do not limit motion in any direction.In fact, ADRs of this kind may not, by themselves, provide sufficientdistraction across the disc space. ADR designs with metal plates andpolyethylene spacers may restrict translation but they do not limit theother motions mentioned above. The articular surface of the poly spaceris generally convex in all directions. Some ADR designs limit motiontranslation by attaching the ADR halves at a hinge.

One of the most important features of an artificial disc replacement(ADR) is its ability to replicate the kinematics of a natural disc. ADRsthat replicate the kinematics of a normal disc are less likely totransfer additional forces above and below the replaced disc. Inaddition, ADRs with natural kinematics are less likely to stress thefacet joints and the annulus fibrosus (AF) at the level of the discreplacement. Replicating the movements of the natural disc alsodecreases the risk of separation of the ADR from the vertebrae above andbelow the ADR.

The kinematics of ADRs are governed by the range of motion (ROM), thelocation of the center of rotation (COR) and the presence (or absence)of a variable center of rotation (VCOR). Generally ROM is limited by thefacet joints and the AF. A natural disc has a VCOR, that is, the CORvaries as the spine bends forward (flexion) and backward (extension).Typically, the vertebra above a natural disc translates forward 1-2 mmas the spine is flexed.

Prior art total disc replacements (TDR), that is, ADRs with rigid platesthat attach to the vertebrae, do not replicate the kinematics of thenatural disc. Generally, the COR lies too anterior. Most prior art TDRsalso rely on a single, fixed COR. As a result, many of the prior artTDRs have a limited ROM.

SUMMARY OF THE INVENTION

This invention improves upon prior art TDRs by more closely replicatingthe kinematics of a natural disc. The preferred embodiments feature twoor more fixed centers of rotation (CORs) and an optional variable COR(VCOR) as the ADR translates from a fixed posterior COR to a moreanterior COR.

The multiple CORs permit a TDR with a posterior COR that lies posteriorto the COR of the TDR to facilitate normal disc motion. The use of twoor more CORs allow more flexion and more extension than permitted by thefacet joints and the AF. Artificial facet joint-like components may alsobe incorporated into the design to restrict excessive translation,rotation, and/or lateral bending.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sagittal cross section of a total disc replacement (TDR)according to the invention having three fixed centers of rotation(CORs);

FIG. 2 is a sagittal cross section of the TDR of FIG. 1 extended 5degrees, more or less;

FIG. 3 is a sagittal cross section of the TDR of FIG. 1 showing variousdegrees of flexion;

FIG. 4 is a sagittal cross section of another embodiment of a TDR havingan anterior COR and a posterior COR;

FIG. 5 is a sagittal cross section of the TDR of FIG. 4 in a flexedposition;

FIGS. 6A and 6B are drawings that show the articulating surfaces of theTDR drawn in FIG. 4;

FIG. 7 is a sagittal cross section of another embodiment having ananterior and a posterior COR;

FIG. 8 is a sagittal cross section of the TDR of FIG. 7 in a more flexedposition;

FIG. 9 is a view of the articulating surfaces of the TDR of FIG. 7;

FIG. 10 is an oblique view of the assembled TDR drawn in FIG. 7;

FIG. 11A is a view of the anterior side of a cervical embodiment of theTDR of FIG. 7;

FIG. 11B is a view of the lateral side of the TDR of FIG. 11A;

FIG. 11C is a view of the interior of the TDR drawn in FIG. 11A;

FIG. 12A is a sagittal cross section of yet a further embodiment of anartificial disc replacement according to the invention;

FIG. 12B is a sagittal cross section of the embodiment of the ADR ofFIG. 12A;

FIG. 12C is a view of the side of the ADR of FIG. 12A;

FIG. 13A is a view of the side of the ADR of FIG. 12A including modularshims; and

FIG. 13B is an exploded view of the embodiment of the ADR shown in FIG.13A.

DETAILED DESCRIPTION OF THE INVENTION

My U.S. Provisional Patent Application Ser. No. 60/374,747, incorporatedherein by reference, describes various improved artificial discreplacements (ADRs), including various embodiments that restrict spinalextension, rotation, translation, and/or lateral bending. In onedisclosed configuration, rotation and translocation are limited by a“spoon-on-spoon” type of cooperation. Wedge or trapezoid-shaped ADRs arealso presented to preserve lordosis. Fasteners may be used to fix theADR to upper and lower vertebrae. An optional lip may additionally beprovided to prevent the trapping of soft tissue during the movement froma flexion to neutral position.

The present invention extends such teachings through total discreplacements (TDRs) that more closely replicate the kinematics of anatural disc. The preferred embodiments feature two or more fixedcenters of rotation (CORs) and an optional variable COR (VCOR) as theADR translates from a fixed posterior COR to a more anterior COR. Themultiple CORs permit a TDR with a posterior COR that lies posterior tothe COR of the TDR to facilitate normal disc motion. The use of two ormore CORs allow more flexion and more extension than permitted by thefacet joints and the AF. Artificial facet joint-like components may alsobe incorporated into the design to restrict excessive translation,rotation, and/or lateral bending.

FIG. 1 is a sagittal cross section of a TDR 10 according to theinvention having three fixed CORs 12, 14, and 16. Articulation occurs atthe posterior COR 12 when the spine is in a neutral to extendedposition. FIG. 2 is a sagittal cross section of the TDR drawn in FIG. 1with the ADR 10 extended 5 degrees, more or less. FIG. 3 is a sagittalcross section of the TDR drawn in FIG. 1 in various degrees of flexion.As illustrated in the figure, the COR migrates anteriorly from a moreposterior COR to a more anterior COR as the TDR is flexed.

FIG. 4 is sagittal cross section of another embodiment TDR 110 of theinvention having an anterior COR 112 and a posterior COR 114. In thiscase, the TDR 110 articulates at the posterior COR 114 with the TDR inneutral to extended position. FIG. 5 is a sagittal cross section of theTDR 110 drawn in FIG. 4 in a flexed position. Note that the superior TDRendplate 110 a translates forward from the posterior COR 114 to theanterior COR 112 as the ADR 110 moves from a neutral or extendedposition to a flexed position. FIGS. 6A and 6B are a view of thearticulating surfaces of the TDR 110 drawn in FIG. 4. The inferior TDRendplate 110 b is shown in FIG. 6A, and the inferior surface of thesuperior TDR endplate 110 a is shown in FIG. 6B.

FIG. 7 is a sagittal cross section of a further embodiment of theinvention, including an anterior and a posterior COR 212 and 214,respectively. The design also includes novel artificial facet joint-likecomponents that prevent excessive translation, rotation, or lateralbending. FIG. 8 is a sagittal cross section of the TDR 210 drawn in FIG.7 in a more flexed position. The drawing illustrates a gap between theartificial facet joint-like portions of the device. FIG. 9 is a view ofthe articulating surfaces of the TDR 210 drawn in FIG. 7. The superiorsurface of the inferior TDR endplate 210 b is drawn on the left. FIG. 10is an oblique view of the assembled TDR 210 drawn in FIG. 7. Thisembodiment of the TDR 210 illustrates the use of a toroidal patch andtwo spherical patches to form the anterior articulating surface of thelower plate. The novel torodial-spherical surface facilitates lateralbending.

FIG. 11A is a view of the anterior side of a cervical embodiment of theTDR 210 drawn in FIG. 7. Screws can be inserted through the holes in theTDR 210 to attach the TDR 210 to the vertebrae. A reversible lockingmechanism can be used to prevent the screws from backing out of thevertebrae. FIG. 11B is a view of the lateral side of the TDR 210 drawnin FIG. 11A. FIG. 11C is a view of the anterior of the TDR 210 drawn inFIG. 11A. The superior surface of the inferior component of the TDR isdrawn on the left.

FIG. 12A is a sagittal cross section of another embodiment TDR 310wherein, in contrast to the embodiment of FIG. 7, the articulatingsurfaces of the anterior and/or the posterior CORs are not congruent.The use of non-congruent articulating surfaces uncouples translationfrom rotation. ADRs with non-congruent joint surfaces allow greaterspinal flexion and extension without corresponding subluxation of thevertebrae. The spherical projections from the upper and lower ADRendplates 310 a and 310 b can cooperate to prevent the upper ADRendplate 310 a from translating posteriorly over the inferior ADRendplate 310 b. The drawing illustrates the different radius ofcurvature of the components forming the joint in the posterior aspect ofthe ADR.

FIG. 12B is a sagittal cross section of the embodiment of the ADR 310drawn in FIG. 12A in a flexed position. The drawing illustrates thedifferent radius of curvature of the components forming the joint in theanterior aspect of the ADR 310. FIG. 12C is a view of the side of theADR 310 drawn in FIG. 12A. Artificial facet joint-like components,similar to those drawn in FIG. 7, prevent excessive forward translationof the upper ADR endplate relative to the lower ADR endplate. Theartificial fact joint-like components can also limit axial rotation andlateral bending.

FIG. 13A is a view of the side of the ADR 310 drawn in FIG. 12A, withmodular sims. Modular shims can be used to increase lordosis, or wedgeshape, of the ADR 310. The modular shims can be attached to the top ofthe superior ADR endplate 310 a and/or the bottom of the inferior ADRendplate 310 b. The shims could fasten to the keels of the ADR 310.Alternatively the shims could attach to another part of the ADRendplates 310 a and 310 b. Lastly, the shims could simply lay on the ADRendplates 310 a and 310 b. The shim inventory would include shims withdifferent thickness and different angles. FIG. 13B is an exploded viewof the embodiment of the ADR drawn in 13 A.

Although surfaces depicted herein are shown as being ‘congruent,’ thisis not necessary according to the invention. For example, a concavesurface may have a radius of curvature that is larger than the radius ofcurvature of an articulating convex surface such that the two surfacesare not in direct or intimate contact at all times. Both symmetrical andasymmetrical joints may also be used. A portion of the back of theposterior joint may be removed to move the posterior COR furtherposterior and to increase the surface area of the posterior joint byincreasing the radius of the surface. The articulating surface may beformed by a toroidal region and a spherical region, in this and otherembodiments non-spherical surfaces may also be used to permittranslation, rotation or other movements between more controlledarticulations. TDRs according to the invention may be used in thecervical, thoracic, or lumbar spine.

ADR/TDRs according to this invention may also be composed of variousmaterials. For example, the components may be constructed of a metalsuch as chrome cobalt or a ceramic such as aluminum oxide. The novel TDRcan also be made of a metal or ceramic coated with a harder or softersecond material. That is, one or both of the components may be a metalcoated with a ceramic, or a metal or ceramic coated with a diamond-likematerial or other hardened surface. Alternatively, one or both of thecomponents may be coated with a polymeric (i.e., polyethylene) surfaceor liner.

1. An artificial disc replacement (ADR) configured for positioningbetween vertebral bodies having an anterior portion and a posteriorportion, comprising: a superior component with a lower articulatingsurface; and an inferior component with an upper articulating surfacethat cooperates with the lower surface through a first center ofrotation located above the superior component and a second center ofrotation located below the inferior component, wherein the first centerof rotation is located in the anterior or posterior portion and thesecond center of rotation is located in the other of the anterior orposterior portion.
 2. The ADR of claim 1, wherein the lower articulatingsurface is convex and the upper articulating surface is concave.
 3. TheADR of claim 1, wherein the lower and upper articulating surfaces arecongruent or non-congruent.
 4. The ADR of claim 1, further including oneor more facet approximating features that limit excessive translation,rotation and/or lateral bending.
 5. The ADR of claim 1, furtherincluding projections on the superior and inferior components forengaging the vertebral bodies.
 6. The ADR of claim 5, wherein theprojections are keels.
 7. An artificial disc replacement (ADR) havingtwo centers of rotation (CORs) configured for positioning betweenvertebral bones, each having an anterior portion and a posteriorportion, the ADR comprising: a superior component with a lowerarticulating surface and a first projection; an inferior component withan upper articulating surface and a second projection; and wherein oneof the CORs is located above the superior component and in the anterioror posterior portion, and the other of the CORs is located below theinferior component and in the other of the anterior or posteriorportion.
 8. The ADR of claim 7, wherein the lower articulating surfaceis convex and the upper articulating surface is concave.
 9. The ADR ofclaim 7, wherein the lower and upper articulating surfaces are congruentor non-congruent.
 10. The ADR of claim 7, further including one or morefacet approximating features that limit excessive translation, rotationand/or lateral bending.
 11. The ADR of claim 7, wherein the projectionsare keels.
 12. An artificial disc replacement (ADR) configured forpositioning between vertebral bodies having an anterior portion and aposterior portion, comprising: a superior component with a lowerarticulating surface; and an inferior component with an upperarticulating surface that cooperates with the lower surface through twoor more fixed or variable centers of rotation (CORs), at least one CORlocated above the superior component and at least one COR located belowthe inferior component, wherein at least one COR is located in theanterior or posterior portion and at least one COR is located in theother of the anterior or posterior portion.
 13. The ADR of claim 12,wherein the lower and upper articulating surfaces are congruent ornon-congruent.
 14. The ADR of claim 12, further including at least oneset of articulating surfaces forming a separate center of rotation inthe posterior portion.
 15. The ADR of claim 14, wherein the anterior andposterior centers of rotation are spherical, symmetrical, or both. 16.The ADR of claim 14, wherein the anterior and posterior centers ofrotation are at different vertical heights.
 17. The ADR of claim 12,including a substantially smooth transition between the centers ofrotation.
 18. The ADR of claim 12, further including one or more facetapproximating features that limit excessive translation, rotation and/orlateral bending.
 19. The ADR of claim 12, further including projectionson the superior and inferior components for engaging the vertebralbodies.
 20. The ADR of claim 19, wherein the projections are keels.